Dopant ink composition and method of fabricating a solar cell there from

ABSTRACT

Dopant ink compositions and methods of fabricating solar cells there from are described. A dopant ink composition may include a cross-linkable matrix precursor, a bound dopant species, and a solvent. A method of fabricating a solar cell may include delivering a dopant ink composition to a region above a substrate. The dopant ink composition includes a cross-linkable matrix precursor, a bound dopant species, and a solvent. The method also includes baking the dopant ink composition to remove a substantial portion of the solvent of the dopant ink composition, curing the baked dopant ink composition to cross-link a substantial portion of the cross-linkable matrix precursor of the dopant ink composition, and driving dopants from the cured dopant ink composition toward the substrate.

The invention described herein was made with Governmental support undercontract number DE-FC36-07GO17043 awarded by the United StatesDepartment of Energy. The Government may have certain rights in theinvention.

TECHNICAL FIELD

Embodiments of the present invention are in the field of renewableenergy and, in particular, dopant ink compositions and methods offabricating solar cells there from.

BACKGROUND

Photovoltaic cells, commonly known as solar cells, are well knowndevices for direct conversion of solar radiation into electrical energy.Generally, solar cells are fabricated on a semiconductor wafer orsubstrate using semiconductor processing techniques to form a p-njunction near a surface of the substrate. Solar radiation impinging onthe surface of, and entering into, the substrate creates electron andhole pairs in the bulk of the substrate. The electron and hole pairsmigrate to p-doped and n-doped regions in the substrate, therebygenerating a voltage differential between the doped regions. The dopedregions are connected to conductive regions on the solar cell to directan electrical current from the cell to an external circuit coupledthereto.

Efficiency is an important characteristic of a solar cell as it isdirectly related to the capability of the solar cell to generate power.Likewise, efficiency in producing solar cells is directly related to thecost effectiveness of such solar cells. Accordingly, techniques forincreasing the efficiency of solar cells, or techniques for increasingthe efficiency in the manufacture of solar cells, are generallydesirable. Embodiments of the present invention allow for increasedsolar cell manufacture efficiency by providing novel processes forfabricating solar cell structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a dopant ink composition, inaccordance with an embodiment of the present invention.

FIG. 1B is a schematic diagram of a dopant ink composition, inaccordance with another embodiment of the present invention.

FIG. 2A includes chemical representations of phosphine moieties andorganophosphine moieties for dopant ink compositions, in accordance withan embodiment of the present invention.

FIG. 2B includes chemical representations of borane moieties andorganoborane moieties for dopant ink compositions, in accordance with anembodiment of the present invention.

FIG. 3 is a flowchart representing operations in a method of fabricatinga solar cell, in accordance with an embodiment of the present invention.

FIG. 4A illustrates a cross-sectional view of a stage in the fabricationof a solar cell, corresponding to operation 302 of the flowchart of FIG.3, in accordance with an embodiment of the present invention.

FIG. 4B illustrates a cross-sectional view of a stage in the fabricationof a solar cell, corresponding to operation 304 of the flowchart of FIG.3, in accordance with an embodiment of the present invention.

FIG. 4C illustrates a cross-sectional view of a stage in the fabricationof a solar cell, corresponding to operation 306 of the flowchart of FIG.3, in accordance with an embodiment of the present invention.

FIG. 4D illustrates a cross-sectional view of a stage in the fabricationof a solar cell, corresponding to operation 308 of the flowchart of FIG.3, in accordance with an embodiment of the present invention.

FIGS. 5A and 5B illustrate cross-sectional views of various stages inthe fabrication of a solar cell, in accordance with another embodimentof the present invention.

FIGS. 6A and 6B illustrate cross-sectional views of various stages inthe fabrication of a solar cell, in accordance with another embodimentof the present invention.

DETAILED DESCRIPTION

Dopant ink compositions and methods of fabricating solar cells aredescribed herein. In the following description, numerous specificdetails are set forth, such as specific process flow operations, inorder to provide a thorough understanding of embodiments of the presentinvention. It will be apparent to one skilled in the art thatembodiments of the present invention may be practiced without thesespecific details. In other instances, well-known fabrication techniques,such as metal contact formation techniques, are not described in detailin order to not unnecessarily obscure embodiments of the presentinvention. Furthermore, it is to be understood that the variousembodiments shown in the figures are illustrative representations andare not necessarily drawn to scale.

Disclosed herein are dopant ink compositions. In one embodiment, adopant ink composition includes a cross-linkable matrix precursor, abound dopant species, and a solvent. In another embodiment, a dopant inkcomposition includes a cross-linkable matrix precursor, a plurality ofparticles, a dopant species coupled to one of the particles, and asolvent.

Also disclosed herein are methods of fabricating solar cells. In oneembodiment, a method includes delivering a dopant ink composition to aregion above a substrate, baking the dopant ink composition to remove asubstantial portion of a solvent of the dopant ink composition, curingthe baked dopant ink composition to cross-link a substantial portion ofa cross-linkable matrix precursor of the dopant ink composition, anddriving dopants from the cured dopant ink composition toward thesubstrate.

Dopant inks enable use of liquid deposition methods in solar cellfabrication (and, perhaps, more generally semiconductor structurefabrication) as a route to selectively doping a substrate. Such liquiddeposition methods may offer reduced cost relative to typicalvapor-phase dopant deposition techniques, which typically involveexpensive furnaces and vacuum chambers in addition to toxic gas deliveryand exhaust systems. While dopant inks may be desirable for costreduction, they often out-gas the dopant or dopant precursor includedtherein over a wide temperature range, even as low as 50 degrees Celsiusand up to diffusion temperatures around 1000 degrees Celsius or higher.The dopant or dopant precursor out-gassing may lead to difficulty inachieving precise control of a resulting diffusion profile, and may leadto unintended doping of certain regions of the substrate, e.g., in theform of counter-doping or auto-doping.

For effective implementation of a suitable dopant ink in a solar cellmanufacturing process, tight temperature control has typically beenrequired. Such control of the process has been achieved by laboriousprecise tooling and significant recipe tuning. Furthermore, increasinggas flow or pulling a vacuum during thermal operations may have to beimplemented. The above measures usually come with increased toolrequirements and cost. Additional approaches to address or mitigateout-gassing from dopant inks include capping the dopant ink with anothermaterial, such as an oxide material, a nitride material, or amorphoussilicon. However, the capping approaches require at least one additionaloperation in a process flow and often further involve increased tool andmaterial costs. Also, the cap layer may need to be removed in subsequentprocessing, furthering again the costs involved. Another considerationfor dopant inks comes in a need for efficiency when both types ofdopants, e.g. n-type and p-type, are needed in different regions of areceiving substrate.

In accordance with one or more embodiments of the present invention,issues associated with out-gassing of dopants or dopant precursors fromdopant inks are addressed. In one such embodiment, an approach fordesign of dopant ink formulations which reduces dopant out-gassing byintegrating a dopant or a dopant precursor into a bound state in thematrix of the ink is provided. For example, in a specific embodiment, adopant ink includes a siloxane support matrix, such as is often includedin common spin-on glass (SOG) materials. The dopant ink has siloxanemonomers, which undergo a cross-linking reaction at elevated temperatureor with ultraviolet (UV) light to form a silicon oxide matrix. Alsoincluded in the dopant ink is the bound dopant or dopant precursor,which may be included as such in several different manners, described inmore detail below.

In a first such embodiment, the bound dopant or dopant precursor ischemically bonded to a siloxane backbone during siloxane synthesis. Thisapproach may increases the molecular weight of a support moleculeincluding a dopant, thus decreasing the vapor pressure and thereby thedopant out-gassing. Additionally, as the temperature is increased topromote the siloxane cross-linking reaction, the dopant-siloxanemolecule may become tethered within the matrix. Once the cross-linkingreaction takes place, the dopant may remain bound to the matrix until atemperature is reached sufficient to break the chemical bond of thedopant atom from the siloxane backbone. In one embodiment, the siloxanecross-linking reaction kinetics are altered to favor higher or lowertemperatures by modifying the terminal groups of the siloxane. Theresulting siloxane matrix densifies to form a silicon oxide matrix,which may provide a greater ability to bind dopant atoms than thesiloxane solution of the ink. Thus, by tailoring the reactivity of thesiloxanes, a two-stage dopant retention mechanism may be established.Namely, the dopant is retained at lower temperatures likely due to thechemical binding of the dopant atom to the siloxane backbone, and thedopant is retained at higher temperatures likely due to the binding ofthe dopant atom within the resulting silicon oxide matrix.

In a second such embodiment, the bound dopant or dopant precursor isincorporated within a particle, such as a nanoparticle, blended in asuspension with the siloxane monomers. Such particles may includesilicon oxide based nanoparticles or silicon nanoparticles. By bindingthe dopant atom within a solid, the diffusion coefficient of the dopantatom may be reduced as compared to the dopant remaining in solution,e.g., an essentially free form counterpart. The effect may beparticularly pronounced at lower temperatures, e.g., during a bakeprocess, an organic burnout process, or initial start of a temperatureramp toward a diffusion temperature. The siloxane matrix may densify atelevated temperatures to form a silicon oxide matrix, which provides agreater ability to bind dopant atoms than the siloxane solution of theink. In the case of a suspension of nanoparticles within a siloxanemonomer liquid phase, a two-stage dopant retention mechanism may beprovided. Namely, the dopant is retained at lower temperatures likelydue to the binding of the dopant atom within the solid-phasenanoparticle, and the dopant is retained at higher temperatures likelyby way of the additional binding created by the nanoparticles residingwithin the silicon oxide matrix.

Embodiments of the present invention may be implemented in a variety ofsolar cell technology platforms. In one embodiment, a dopant inkcomposition is used as either a p-type or n-type dopant source. Inanother embodiment, a dopant ink composition is used as one or both ofthe p-type and n-type dopant sources.

In an aspect of the present invention, a dopant ink composition issuitable for, upon depositing on a substrate or layer, delivering chargecarrier dopant atoms, e.g., for fabrication of a solar cell. FIGS. 1Aand 1B are schematic diagrams of dopant ink compositions, in accordancewith an embodiment of the present invention.

Referring to FIGS. 1A and 1B, dopant ink compositions 100A and 100B,respectively, each include a cross-linkable matrix precursor (designatedas X) 102A and 102B, respectively. Bound dopant species (designated asD) 104A and 104B, respectively, are also included. The cross-linkablematrix precursors 102A and 102B and the bound dopant species 104A and104B are included in solvents 106A and 106B, respectively.

Referring only to FIG. 1A, in an embodiment, the bound dopant species104A is bound to the cross-linkable matrix precursor 102A. For example,in one embodiment, the bound dopant species 104A is a dopant precursorbound to the cross-linkable matrix precursor through a chemical bond.The chemical bond may be one in which electrons are shared or donatedbetween an atom of the bound dopant species 104A and an atom of thecross-linkable matrix precursor 102A. In a specific such embodiment, thechemical bond is one such as, but not limited to, a covalent bond, anionic bond, or a dative bond. However, strictly electrostaticinteractions, such as Van der Waals interactions, may not be consideredin the definition of a chemical bond.

In an embodiment, referring again to FIG. 1A, the bound dopant species104A is an n-type dopant precursor such as, but not limited to, aphosphine moiety (e.g., 202 and 204 of FIG. 2A) or an organophosphinemoiety (e.g., 206 and 208 of FIG. 2A, where R, R′ and R″ are organicgroups). It is to be understood that although only moieties withoxidation state III for phosphorus are shown for the n-type dopantprecursor, moieties with oxidation state V for phosphorus are alsocontemplated. In another embodiment, the bound dopant species 104A is ap-type dopant precursor such as, but not limited to, a borane moiety(e.g., 212 and 214 of FIG. 2B) or an organoborane moiety (e.g., 216 and218 of FIG. 2B, where R, R′ and R″ are organic groups).

Referring only to FIG. 1B, in an embodiment, the dopant ink composition100B further includes a plurality of particles 108. The bound dopantspecies 104B is bound to one or more of the particles 108. The particles108 may be, or may be formed from, material species such as, but notlimited to, nanoparticles, flakes, strands, or macromolecules such asproteins, or large organic molecules. In one embodiment, the averagesize of each of the particles 108 is in the nanometer to micron range,e.g., as measured as a diameter of the particle.

In an embodiment, referring again to FIG. 1B, the bound dopant species104B is an n-type or a p-type dopant precursor bound to a surface of oneof the particles through a chemical bond. Such an arrangement may besimilar to the chemical bonding described in association with FIG. 1Aand may include moieties such as those described above in associationwith FIGS. 2A and 2B. In such embodiments, the bound dopant species 104Bmay also be described as being tethered to the particles 108. As analternative embodiment, however, the bound dopant species 104B may be ann-type or a p-type dopant atom (e.g., such as a boron or phosphorousatom) incorporated into one of the particles 108 as an actual componentof the particle 108. In such embodiments, the dopant atoms may bechemically bonded to other atoms in the particles 108, or may beelectrostatically trapped by other atoms in the particles 108, e.g., ascaged or cryptand-bound dopant atoms.

In an embodiment, the cross-linkable matrix precursor 102A or 102B isone that is thermally cross-linkable such as, but not limited to, asiloxane species, a silane species, or a cyclosilane species. In anembodiment, the solvent 106A or 106B is an organic solvent, such as butnot limited to, an alkyl-based alcohol (e.g., ethanol or propanol), anaryl-based alcohol (e.g., phenol), decalin, an alkane (e.g., hexanes,cyclohexanes, octanes), an aromatic (e.g., toluene or otherbenzene-derivatives) an ethyl acetate species (e.g., propylene glycolmethyl ethyl acetate (PGMEA)). However, in another embodiment, thesolvent 106A or 106B is aqueous-based and includes water (e.g., smallsilane species or small siloxane species as cross-linkable matrixprecursors may be compatible with aqueous-based solvents). In anembodiment, the cross-linkable matrix precursor 102A or 102B isdissolved in the solvent 106A or 106B.

In another aspect of the present invention, a solar cell may befabricated by forming doped regions with a dopant ink composition. Forexample, FIG. 3 is a flowchart 300 representing operations in a methodof fabricating a solar cell, in accordance with an embodiment of thepresent invention. FIGS. 4A-4D illustrate cross-sectional views ofvarious stages in the fabrication of a solar cell, corresponding tooperations of flowchart 300, in accordance with an embodiment of thepresent invention.

Referring to operation 302 of flowchart 300, and corresponding FIG. 4A,a method of fabricating a solar cell includes delivering a dopant inkcomposition 404 to a region above a substrate 402.

In an embodiment, the dopant ink composition 404 includes across-linkable matrix precursor, a bound dopant species, and a solvent.In one such embodiment, the dopant ink composition is one of or issimilar to the dopant ink compositions 100A and 100B, described above.For example, in a specific such embodiment, the dopant ink composition404 further includes a plurality of particles, and the bound dopantspecies is coupled to one of the particles. In an embodiment, deliveringthe dopant ink composition 404 to the region above the substrate 402includes using a fluid deposition technique such as, but not limited to,ink jetting, screen printing, blading, transferring by pipette, spincoating and etching, gravure printing, or slot-die coating.

Referring to operation 304 of flowchart 300, and corresponding FIG. 4B,the method also includes baking the dopant ink composition 404 toprovide a baked dopant ink composition 406.

In an embodiment, baking the dopant ink composition 404 removes asubstantial portion of a solvent of the dopant ink composition 404. Inone such embodiment, the solvent is one such as or similar to thesolvents described in association with solvents 106A and 106B above. Inan embodiment, baking the dopant ink composition 404 includes heating toa temperature approximately in the range of 100-400 degrees Celsius.

Referring to operation 306 of flowchart 300, and corresponding FIG. 4C,the method also includes curing the baked dopant ink composition 406 toprovide a cured dopant ink composition 408.

In an embodiment, curing the baked dopant ink composition 406cross-links a substantial portion of a cross-linkable matrix precursorof the dopant ink composition 404. In one such embodiment, thecross-linkable matrix precursor is one such as or similar to thecross-linkable matrix precursors described in association withcross-linkable matrix precursors 102A and 102B above. In an embodiment,curing the baked dopant ink composition 406 includes heating to atemperature approximately in the range of 350-900 degrees Celsius. In anembodiment, the baking of operation 304 and the curing of operation 306are performed in the same process operation.

Referring to operation 308 of flowchart 300, and corresponding FIG. 4D,the method also includes driving dopants from the cured dopant inkcomposition 408 toward the substrate 402.

In an embodiment, driving dopants from the cured dopant ink composition408 includes driving dopant impurity atoms into the substrate 402 toform doped regions 410, e.g., n-type or p-type doped regions, in thesubstrate 402. Thus, in an embodiment, the region of the substrate 402is an upper surface of the substrate 402, and driving dopants from thecured dopant ink composition 408 toward the substrate 402 includesdriving dopants into the substrate 402. In one such embodiment, thedriving includes both migrating and incorporating the dopant impurityatoms into regions 410 of the substrate 402. In an embodiment, drivingdopants from the cured dopant ink composition 408 includes heating to atemperature approximately in the range of 850-1050 degrees Celsius. Inan embodiment, the remaining components of the cured dopant inkcomposition 408 are removed subsequent to the driving, e.g., by a wetetch process.

In an embodiment, substrate 402 is a bulk silicon substrate, e.g., abulk n-type silicon substrate. In one such embodiment, the dopantimpurity atoms are p-type for silicon (such as boron impurity atoms) orare n-type for silicon (such as phosphorus impurity atoms).

FIGS. 5A and 5B illustrate cross-sectional views of various stages inthe fabrication of a solar cell, in accordance with another embodimentof the present invention. Referring to the operations of flowchart 300,in an embodiment, the dopant ink composition 404 is a first dopant inkcomposition for a first conductivity type and the method furtherincludes delivering a second dopant ink composition for a second,different, conductivity type to a second region above the substrate 402.As an example, FIG. 5A depicts a first dopant ink composition 504A for afirst conductivity type and a second dopant ink composition 504B for asecond, different, conductivity type disposed above a substrate 502. Asshown, both the first dopant ink composition 504A and the second dopantink composition 504B have been baked and cured.

Referring to FIG. 5B, dopants are driven from the cured first dopant inkcomposition 504A and the cured second dopant ink composition 504B towardthe substrate 502. In an embodiment, driving dopants from cured dopantink compositions includes driving dopant impurity atoms into thesubstrate 502 to form doped regions 510A and 510B, e.g., n-type dopedregions 510A and p-type doped regions 510B or alternatively p-type dopedregions 510A and n-type doped regions 510B, in the substrate 502.

In an embodiment, the second dopant ink composition 504B is deliveredprior to baking the first dopant ink composition 504A. In one suchembodiment, the first dopant ink composition 504A and the second dopantink composition 504B are delivered sequentially. In another suchembodiment, the first dopant ink composition 504A and the second dopantink composition 504B are delivered substantially simultaneously.

In another embodiment, however, the second dopant ink composition 504Bis delivered subsequent to baking but prior to curing the first dopantink composition 504A. The second dopant ink composition 504B is thenindependently baked, while curing and driving the dopants from the firstand second dopant ink compositions is performed in the same sets ofprocess operations, respectively. In yet another embodiment, the seconddopant ink composition 504B is delivered subsequent to baking andcuring, but prior to driving dopants from, the first dopant inkcomposition 504A. The second dopant ink composition 504B is thenindependently baked and cured, while driving the dopants from the firstand second dopant ink compositions is performed in the same processoperation. In an alternative embodiment, all of baking, curing anddriving dopants from the second dopant ink composition 504B is doneindependently from baking, curing and driving dopants from the firstdopant ink composition 504A.

FIGS. 6A and 6B illustrate cross-sectional views of various stages inthe fabrication of a solar cell, in accordance with another embodimentof the present invention. Referring to the operations of flowchart 300,in an embodiment, the dopant ink composition 404 is formed on an uppersurface of a semiconductor layer disposed above the substrate 402. As anexample, FIG. 6A depicts a dopant ink composition 604 disposed above asemiconductor layer 620 disposed above a substrate 602. In oneembodiment, a thin dielectric layer 622 is disposed between thesemiconductor layer 620 and the substrate 602, as depicted in FIG. 6A.

Referring to FIG. 6B, driving dopants from the dopant ink composition604 (e.g., from a cured form of a dopant ink composition) toward thesubstrate 602 involves driving dopants into the semiconductor layer 620.In an embodiment, driving dopants from the dopant ink composition 604includes driving dopant impurity atoms into the semiconductor layer 620to form doped regions 610, e.g., n-type or p-type doped regions, in thesemiconductor layer 620.

In an embodiment, semiconductor layer 620 is an amorphous orpolycrystalline silicon layer. In one such embodiment, the dopantimpurity atoms are p-type for silicon (such as boron impurity atoms) orare n-type for silicon (such as phosphorus impurity atoms). In aspecific such embodiment, substrate 602 is a bulk silicon substrate,e.g., a bulk n-type silicon substrate, and thin dielectric layer 622 isa silicon oxide or silicon dioxide layer.

Thus, dopant ink compositions and methods of fabricating solar cellsthere from have been disclosed. In accordance with an embodiment of thepresent invention, a dopant ink composition includes a cross-linkablematrix precursor, a bound dopant species, and a solvent. In oneembodiment, the bound dopant species is bound to the cross-linkablematrix precursor. In one embodiment, the dopant ink composition furtherincludes a plurality of particles, and the bound dopant species is boundto one or more of the particles. In accordance with another embodimentof the present invention, a method of fabricating a solar cell includesdelivering a dopant ink composition to a region above a substrate. Thedopant ink composition includes a cross-linkable matrix precursor, abound dopant species, and a solvent. The method also includes baking thedopant ink composition to remove a substantial portion of the solvent ofthe dopant ink composition, curing the baked dopant ink composition tocross-link a substantial portion of the cross-linkable matrix precursorof the dopant ink composition, and driving dopants from the cured dopantink composition toward the substrate.

What is claimed is:
 1. A dopant ink composition, comprising: a solvent;a cross-linkable matrix precursor comprising siloxane monomers dispersedin the solvent, one or more of the siloxane monomers bound to a dopantspecies through a chemical bond; and silicon nanoparticles or siliconoxide nanoparticles.
 2. The dopant ink composition of claim 1, whereinthe dopant species is an n-type dopant precursor.
 3. The dopant inkcomposition of claim 2, wherein the n-type dopant precursor is selectedfrom the group consisting of a phosphine moiety and an organophosphinemoiety.
 4. The dopant ink composition of claim 1, wherein the solvent isselected from the group consisting of an alkyl-based alcohol, anaryl-based alcohol, decalin, an alkane, an aromatic, an ethyl acetatespecies, and water, and wherein the cross-linkable matrix precursor isdissolved in the solvent.
 5. The dopant ink composition of claim 1,wherein the chemical bond is selected from the group consisting of acovalent bond, an ionic bond, and a dative bond.
 6. The dopant inkcomposition of claim 1, wherein the dopant species is a p-type dopantprecursor.
 7. The dopant ink composition of claim 6, wherein the p-typedopant precursor is selected from the group consisting of a boranemoiety and an organoborane moiety.